Haloalkylation, generally chloromethylation, is a well known reaction which is an important step in the preparation of anion-exchange resins. It allows the introduction of reactive sites on which ion-exchange groups will attach in the resin matrix. It follows from this that the ion-exchange capacity of the resin obtained will depend upon the number of haloalkyl sites introduced into the skeleton.
Generally, the chloromethylation process is carried out in a nonaqueous medium by reacting chloromethyl methyl ether (CH.sub.3 OCH.sub.2 Cl), pure or mixed with a solvent, on bead polymers or copolymers at temperatures between about 0.degree. and 70.degree. C. and with reaction times ranging from 0.5 to 8 hours. The reaction is catalyzed by Friedel-Crafts type compounds, protonic or Lewis acids.
Another process for chloromethylation of bead copolymers or polymers consists of using reaction mixtures which produce chloromethyl methyl ether in situ. These reaction mixtures may be, for example, mixtures of paraformaldehyde and hydrochloric acid; mixtures of formaldehyde polymers and aluminum chloride; mixtures of methylal, thionyl chloride and catalyst; mixtures of methylal, sulfuryl chloride plus chlorosulfonic acid, or chlorosulfonic acid only; and mixtures of methylal, chlorosulfonic acid, and an oxygen-containing polar liquid. Such reaction mixtures are also known as chloromethylating complexes. Among these chloromethylating complexes the most commonly used are the ones obtained from chlorosulfonic acid. The reaction may be represented as: EQU CH.sub.3 OH+CHOH+SO.sub.3 HCl.fwdarw.CH.sub.3 OCH.sub.2 Cl+H.sub.2 SO.sub.4
The advantages of using chloromethylating complexes are well-known, and include no direct handling of chloromethyl methyl ether which is a very toxic compound; no separation operation, i.e. the complex is a homogeneous liquid miscible with most of the organic solvents which can be used simultaneously during the chloromethylation; and limitation of the number of reactors required which results in a reduction in equipment investment. In addition, particularly in the case of the preparation of complexes leading to sulfuric acid formation, the catalyst is produced at the same time as the chloromethylating compound. Therefore the quantity of Lewis acid type catalyst required for the chloromethylation reaction is reduced to a large degree.
However the drawbacks resulting from the use of complexes containing large quantities of sulfuric acid are also well known. Too great an excess of sulfuric acid leads to the formation of large amounts of waste sulfuric acid which presents corrosion and pollution problems. Large excesses of sulfuric acid also lead to the formation of methylic bridges between the aromatic rings in the polymer skeleton, resulting in appreciable additional cross-linking in comparison to the initial cross-linking of the polymer or copolymer used. This reduces the number of haloalkyl groups introduced and thus the number of available sites for further reaction with the ion-exchange functional group. The ion-exchange resins obtained are dense and less porous.
In some cases this additional cross-linking may be desirable, since a higher cross-linking rate modifies the final ion-exchange characteristics such as capacity and dry matter. However, until now it has not been possible to regulate this additional cross-linking rate.
Therefore, an object of the present invention is to provide a process of chloromethylation for cross-linked vinylaromatic polymers or copolymers using a chloromethylating complex. This process permits a reduction in or control of the additional cross-linking obtained where using such a complex.